The present invention relates to a dielectric antenna adapted to communicate signals in dual band with one antenna and to a communication device incorporating such an antenna. More particularly, the present invention relates to a dielectric antenna which is loaded on personal computers, cellular phones, portable remote terminals and so forth, suitable for use in LANs (Local Area Networks).
Utilization of wireless LAN becomes popular in recent years, the wireless LAN using radio waves for exchanging data between units of electronic equipment; for example, among personal computers and between a personal computer and a cellular phone. For the wireless LAN, there have heretofore been used a frequency band of 2.4 GHz only and a dielectric antenna as an antenna generally employing a dielectric substrate and a radiation electrode formed with a conductive film for downsizing purposes.
With the recent development of information technology, the data exchanged over the wireless LAN has come to include data such as images having a large quantity of information. Consequently, it is proposed to use different frequency bands; namely, a frequency band of 5.2 GHz so that data having a large quantity of information is communicated at a high transmission rate and a frequency band of 2.4 GHz offering a long communication distance so that ordinary data is communicated out of the information communicated over the wireless LAN. Therefore, with respect to the antenna for the wireless LAN loaded on electrical equipment having the radio communication functions of the sort mentioned above, it is also conceivable to juxtapose a first antenna for the 2.4 GHz band (size: 15 mm (length)×7 mm (width)×6 mm (height)) and a second antenna for the 5.2 GHz band (size: 10 mm (length)×4 mm (width)×3 mm (height)).
On the other hands, there is a known method of making one antenna trigger resonances in two desired frequency bands by using a folded element (in a meandering form) to form a radiation electrode with a conductive film so as to adjust the number of meanderings as well as the element-to-element distance (cf., Japanese Patent Publication No. 10-13135A, for example).
Further, as shown in FIG. 12, there is a known antenna of a one-chip type corresponding to dual band and having a feeder-side radiation electrode 53 and a non-feeder-side radiation electrode 54 that are formed side by side on the top face of a rectangular dielectric substrate 51 such that excitation directions A and B cross at right angles (cf., Japanese Patent Publication No. 2001-7639A, for example).
With the above configuration that two antennas are juxtaposed, two antennas have to be produced for each set of electrical equipment and this results in an increase in the cost. Since the two antennas have to be juxtaposed, they take up a lot of space and this is contrary to the demands of the present age for the downsizing of electronic equipment.
On the other hand, in order to make one radiation electrode trigger resonances in dual band such that one is about twice as long as the other, a meticulous adjustment becomes required and this also results in an increase in the cost because the adjustment of resonance frequency on one side to deal with anomalies in products affects the resonance frequency and matching characteristic in the other frequency band, so that an increase in the cost occurs as the adjusting man-hour increases.
In such a type that two radiation electrodes are formed on the same face of a dielectric substrate, the surface area of an antenna tends to become large because two of the radiation electrodes are disposed in parallel on the face side of the dielectric substrate and this results in failing to meet the demands for downsizing. Even though these radiation electrodes are arranged so that the excitation directions cross at right angles, the problem in this case is that the electrodes will interfere with each other when the space therebetween is narrow and the adjustment of the resonance frequency on one side affects the matching characteristic and resonance frequency in the other frequency band, thus making the adjustment difficult. Although the space between the radiation electrodes has to be increased to avoid the above isolation problem as much as possible, another problem arising from increasing the space between the two radiation electrodes is that the surface area of the antenna becomes still larger.